Modified natural rubber particle, production method thereof, and modified natural rubber latex

ABSTRACT

A modified natural rubber particle having carbon-carbon double bands assigned to multifunctional vinyl monomers graft-copolymerized onto the surface of the modified natural rubber particle, which is prepared by graft-copolymerization of multifunctional vinyl monomers having two or more carbon-carbon double bonds onto natural rubber particles or deproteinized natural rubber particles. One of the methods for producing a modified natural rubber particle includes the steps of: forming inclusion complex of the multifunctional vinyl monomers having two or more carbon-carbon double bonds with a guest-protecting agent to protect at least one of the double bonds of the multifunctional vinyl monomers; graft-copolymerizing the resulting inclusion complex onto natural rubber particles or deproteinized natural rubber particles; and deprotecting the protected double bonds by removing the guest-protecting agent from the obtained graft-copolymer.

This is a Division of application Ser. No. 13/001,487 filed Dec. 27,2010, which in turn is a 35 U.S.C. 371 of PCT/JP2009/062530, filed Jul.9, 2009. The disclosure of the prior applications is hereby incorporatedby reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to natural rubber particle modified bygrafting a multifunctional vinyl monomer, a production method thereof,and a modified natural rubber latex.

BACKGROUND ART

Natural rubber has excellent properties such as high tensile strengthand small heat buildup caused by vibration. Therefore, natural rubberhas been utilized in a wide variety of applications such as a tire, abelt and a rubber glove. In addition, in view of environmentalprotections such, as the conservation of resources and the reduction ofcarbon dioxide emissions, natural rubber is attracting attention as analternative to synthetic rubber.

However, natural rubber is also known to be inferior in heat resistance,oil resistance, ozone resistance, etc. in comparison with, syntheticrubber. Therefore, in order to superadd excellent properties to naturalrobber without sacrificing the outstanding properties of natural rubber,attempts to modify natural rubber through grafting, epoxidation,hydrogenation, etc. have been made thus far. For example, in PatentCitations 1 and 2, a technology for graft-copolymerizing a monomer withan unsaturated bond onto deproteinized natural rubber is introduced. Inaddition, in Patent Citation 3, a technology for forming crosslinkingjunctions between rubber molecules through a reaction, of a monomerhaving two or more vinyl groups to natural rubber latex is introduced.

CITATION LIST

-   Patent Citation 1: Publication of Japanese Patent No. 3294903-   Patent Citation 2: Publication of Japanese Patent No. 4025868-   Patent Citation 3: Publication of Japanese Patent Application No.    2003-12736

SUMMARY OF THE INVENTION Technical Problem

In Patent Citations 1 and 2, a monomer used for thegraft-copolymerization onto deproteinized natural rubber has only onecarbon-carbon double bond (C═C) (for example, methyl methacrylate andstymie). This double bond, is consumed during graft-copolymerization.Therefore, on the surface of the grafted natural rubber particles, thereis no carbon-carbon double bond derived from the monomer. In otherwords, a grafted polymer on the surface of the natural rubber particleshas no reactive site. Accordingly, the grafted polymers cannot bechemically bonded to each other by crosslinking. In addition, thedevelopment of a new function through an interaction and/or reactionwith fillers or additives blended during processing the rubber cannot beexpected.

Meanwhile, according to Patent Citation 3, a monomer having two or morevinyl groups is reacted with natural rubber latex. The monomer used inthe reaction has two or more carbon-carbon double bonds. However, almostall vinyl groups (carbon-carbon double bonds) are consumed, due tografting the monomer onto natural rubber particles and formingcrosslinking junctions. Therefore, the grafted polymer on the surface ofthe grafted natural rubber particles has no reactive site. In addition,in the modified rubber latex described in Patent Citation 3, the naturalrubber particles are covered with a grafted polymer layer in which themonomers are polymerized. In addition, between rubber molecules,crosslinking junctions are formed. Therefore, for example, when theamount of the monomer used for the grafting increases, a strain occursbetween the grafted polymer and the natural rubber particles, which maycause a problem with a film-forming property. The grafted polymer layerformed by the method described in Patent. Citation 3 is easily destroyedin the course of processing such as rubber kneading. Therefore, it isdifficult to process the modified natural rubber latex, and the effectsdue to the modification are reduced.

The present invention was devised in view of the above, and the presentinvention provides a modified natural rubber particle that is highlyreactive by retaining carbon-carbon double bonds derived fromgraft-copolymerized monomers on the surface of the modified naturalrubber particle, and a modified natural rubber latex containing themodified natural rubber particle. Moreover, the present inventionprovides a production method of the modified natural rubber particles.

Solution to Problem

(1) According to a first aspect of the present invention, a modifiednatural rubber particle has carbon-carbon double bonds assigned tomultifunctional vinyl monomers graft-copolymerized onto the surface ofthe modified natural rubber particle, which is prepared bygraft-copolymerization of multifunctional vinyl monomers having two ormore carbon-carbon double bonds onto natural rubber particles ordeproteinized natural rubber particles.

On the surface of the modified natural rubber particle according to thefirst aspect of the present invention, carbon-carbon double bonds remainin the multifunctional vinyl monomers, thus graft-copolymerized. Thecarbon-carbon double bonds become reactive sites for crosslinking etc.Accordingly, for example, by subjecting a solid containing the modifiednatural rubber particle to photocrosslinking etc., a three dimensionalnetwork structure can be formed in a grafted polymer layer. By thistreatment, grafted polymers are strongly bonded to each other through acovalent bond. As a result, it is possible to improve tensile propertyand other properties of the rubber material after the crosslinking. Inaddition, the grafted polymer layer is hardly destroyed by deformation.Therefore, the dimensional stability of the rubber material after thecrosslinking is also improved.

Further, due to the presence of the carbon-carbon double bonds on thesurface, in the case of mixing the modified natural rubber particleaccording to the first aspect of the present invention with other rubbermaterials or fillers, the compatibility may be improved. With respect tothe modified natural rubber particle according to the first aspect ofthe present invention, heat resistance etc., which have been a problemparticular to the natural rubber, is improved by grafting themultifunctional vinyl monomers. Thus, the modified natural rubberparticle according to the first aspect of the present invention isuseful as an alternative to synthetic rubber.

(2) According to a second aspect of the present invention, a modifiednatural rubber latex contains the modified natural rubber particleaccording to the first aspect of the present invention.

The modified natural rubber latex according to the second aspect of thepresent invention is highly reactive and is well compatible with otherrubber materials or fillers. In addition, with the modified naturalrubber latex according to the second aspect of the present invention,various rubber products satisfying the strength required can beproduced.

(3) According to a third aspect of the present invention, a first methodfor producing a modified natural rubber particle having carbon-carbondouble bonds assigned to multifunctional vinyl monomersgraft-copolymerized onto the surface of the modified natural rubberparticle includes the steps of: forming inclusion complex, of themultifunctional vinyl monomers having two or more carbon-carbon doublebonds with a guest-protecting agent to protect at least one of thedouble bonds of the multifunctional vinyl monomers; graft-copolymerizingthe resulting inclusion complex, onto natural rubber particles ordeproteinized natural rubber particles; and deprotecting the protecteddouble bonds by removing the guest-protecting agent from the obtainedgraft-copolymer.

When the multifunctional vinyl monomers having two or more carbon-carbondouble bonds are graft-copolymerized as it is onto the natural rubberparticles, it is difficult to retain the carbon-carbon double bonds onthe surface of the natural rubber particles. This is because, when thereactivities of the carbon-carbon double bonds in the multifunctionalvinyl monomers are the same as each, other, it is difficult to controlthese reactivities, and so during the grafting, all of the double bondsreact.

In this point, by the first production method according to the thirdaspect of the present invention, at least one of the carbon-carbondouble bonds in the multifunctional vinyl monomers is protected with theguest-protecting agent beforehand. By graft-copolymerizing the inclusioncomplex, in which a part of the double bonds is protected, onto thenatural rubber particles etc., the grafting can be performed whileretaining a part of the double bonds (that is, while not reacting a partof the double bonds). Accordingly, by the first production methodaccording to the third aspect of the present invention, even when thereactivities of the carbon-carbon double bonds are the same as eachother, the modified natural rubber particle having the carbon-carbondouble bonds in the graft-copolymerized multifunctional vinyl monomerson the surface thereof can be reliably obtained.

(4) According to a fourth aspect of the present invention, a secondmethod for producing a modified natural rubber particle havingcarbon-carbon double bonds assigned to multifunctional vinyl monomers asa source for graft-copolymerization on the surface of the modifiednatural rubber particle includes the step of graft-copolymerizingmultifunctional vinyl monomers having two or more carbon-carbon doablebonds, at least one of which is distinguished in reactivity from theother double bonds, onto natural rubber particles or deproteinizednatural rubber particles.

In the second method according to the fourth aspect of the presentinvention, multifunctional vinyl monomers having carbon-carbon doublebonds that are distinguished, in reactivity from each other are used.Double bonds of carbon having a low reactivity are not reacted and tend,to remain daring the graft-copolymerization. Therefore, by the secondproduction method according to the fourth aspect of the presentinvention, the modified natural rubber particle having the carbon-carbondouble bonds in the multifunctional vinyl monomers graft-copolymerizedonto the surface thereof can be obtained without protecting the doublebonds. Accordingly, the second production method can reduce theproduction processes in comparison with the first production methodaccording to the third aspect of the present invention. In other words,by the second production method according to the fourth aspect of thepresent invention, the above modified natural rubber particle accordingto the first aspect of the present invention can be produced more simplyand at low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual scheme showing one example of an inclusioncomplex formation step in a first production method of the present,invention (using 1,9-nonanedioldimethacrylate (MDMA) andβ-cyclodextrin);

FIG. 2 is a conceptual scheme showing one example of a grafting step inthe above production method;

FIG. 3 is a conceptual scheme showing one example of a deprotection stepin the above production method;

FIG. 4A is a proton nuclear magnetic resonance (¹H-NMR) spectrum for adeproteinized natural rubber;

FIG. 4B is a ¹H-NMR spectrum for NDMA;

FIG. 4C is a ¹H-NMR spectrum for a graft-copolymer produced by graftpolymerizing NDMA without using a guest-protecting agent:

FIG. 4D is a ¹H-NMR spectrum for the copolymer of Example 2 (NDMA);

FIG. 5 is a graph showing a relationship between the temperature and thestorage normal modulus (E′) for the copolymer of Example 1;

FIG. 6 is a graph showing a relationship between the temperature and thestorage normal modulus (E′) for the copolymer of Example 2;

FIG. 7 is a graph showing a relationship between the temperature and thestorage normal modulus (E′) for the copolymer of Example 3;

FIG. 8A is a ¹H-NMR spectrum for deproteinized rubber;

FIG. 8B is a ¹H-NMR spectrum for 3-acryloyloxy-2-hydroxypropylmethacrylate (GAM);

FIG. 8C is a ¹H-NMR spectrum for the copolymer of Example 7 (GAM);

FIG. 9 is an image obtained by scanning probe microscopy (SPM) fordeproteinized natural rubber; and

FIG. 10 is an SPM image of a copolymer of Example 10.

EMBODIMENTS OF THE INVENTION

Hereinafter, a modified natural rubber particle, a modified naturalrubber latex and a production method of the modified natural robberparticle according to the present invention will be described in detail.

<Modified Natural Rubber Particle>

The modified natural rubber particle according to the present inventionhas carbon-carbon double bonds assigned to multifunctional vinylmonomers graft-copolymerized onto the surface of the modified naturalrubber particle, which is prepared by graft-copolymerization ofmultifunctional vinyl monomers having two or more carbon-carbon doublebonds onto natural rubber particles or deproteinized natural rubberparticles.

In the production of the modified natural rubber particle according tothe present invention, any one of natural rubber and deproteinizednatural rubber in which proteins has been removed may be used. When thedeproteinized natural rubber is used, the reaction rate in thegraft-copolymerization can be improved.

As the natural rubber, for example, a field latex, or a commerciallyavailable ammonia-treated latex etc. may be used. In addition,deproteinization of the natural rubber may adopt various known methods.Examples of the method include (i) a method for decomposing proteins byadding protease or a bacteria to a natural rubber latex (see JapanesePatent Application Publication No. JP-A-6-56902), (ii) a method forcleaning a natural, rubber latex repeatedly using a surfactant such as asoap, and (iii) a method of adding a protein modifier selected from thegroup consisting of urea-based compounds represented by the followinggeneral formula (1) and NaClO to the natural rubber latex and removingthe modifier after modification-treating the proteins in the latex (seeJapanese Patent Application Publication No. JP-A-2004-99696).RNHCONH₂  (1)

(where, R represents H or an alkyl group having 1 to 5 carbon atoms)

The multifunctional vinyl monomers used for the graft-copolymerizationonto the natural rubber particles etc. have two or more carbon-carbondouble bonds. “Vinyl monomers” of the present specification meanmonomers having a vinyl structure. The vinyl structure includes, besidesa vinyl group (—CH═CH₂), a mode in which hydrogen atoms in a vinyl groupare replaced by substituents such as a methyl group (—CH₃). As themultifunctional vinyl monomers, for example, one or more chosen from thefollowing (a) to (c) may be used. Here, “multifunctional (meth)allylcompound” in (b) is a generic name of a multifunctional allyl compound,and a multifunctional methylallyl compound.

-   (a) a multifunctional vinyl monomer derived from a polyalcohol and    an unsaturated carboxylic acid-   (b) a multifunctional vinyl monomer derived from a polyalcohol and a    multifunctional (meth)allyl compound-   (c) a styrene-based multifunctional vinyl monomer

Examples of the multifunctional vinyl monomer of (a) include1,9-nonanedioldimethacrylate (hereinafter, referred to as “NDMA”)represented by the following structural formula (2),3-acryloyloxy-2-hydroxypropyl methacrylate (hereinafter, referred to as“GAM”) represented by the following structural formula (3) and glycidylundecylate methacrylic acid-adduct represented by the followingstructural formula (4).

glycidyl undecylate methacrylic Acid-Adduct

Examples of (a) include, besides the above compounds, di(meth)acrylatessuch as: polyalkylenediol di(meth)acrylates represented by1,10-decanediol di(meth)acrylate; polyalkylene glycol di(meth)acrylatesrepresented by polytetramethylene glycol di(meth)acrylate; ethyleneoxide-propylene oxide-modified bisphenol di(meth)acrylates; and ethyleneoxide-propylene oxide-modified alkyl di(meth)acrylates. In addition,examples of (a) further include tri(meth)acrylates produced by usingglycerin or trimethylol propane as a polyalcohol, tetra(meth)acrylatesproduced by using pentaerythritol as a polyalcohol, and modifiedtris-((meth)acryloxyethyl)isocyanurates.

Examples of the multifunctional vinyl monomers of (b) include2,2-bis(allyloxymethyl)-1-butanol, diallyl ether, glycerol α,α′-diallylether, allyloxy polyethylene glycol-polypropylene glycolmono(meth)acrylates, allyloxy polyalkylene glycol mono(meth)acrylatesand allyloxy polyalkyl alcohol mono(meth)acrylates.

Examples of the multifunctional vinyl monomers of (c) includep(m)-divinyl benzene, 1,4-diisopropenyl benzene,bis(vinylphenyl)alkanes, bis(vinylphenyl)alkylene glycols,(vinylphenyl)allylalkanes and (vinylphenyl)allylalkylene glycols.

Among these multifunctional vinyl monomers, for example GAM has afunctional group (hydroxyl group: —OH) in the molecule thereof.Therefore, when GAM is graft-copolymerized, not only a carbon-carbondouble bond, but also a hydroxyl group becomes a reactive site on thesurface of the particles. Accordingly, a crosslinking reaction utilizingthe hydroxyl group also becomes possible. Thus, by using multifunctionalvinyl monomers having functional groups, the reactive site increases andit can also be expected to impart new functions based on functionalgroups, for example improving gas-transmission resistance.

The graft-copolymerization of multifunctional vinyl monomers onto thenatural rubber particles or the deproteinized natural rubber particleswill be described, in detail in the following descriptions of theproduction method. In addition, the presence or absence of acarbon-carbon double bond derived from multifunctional vinyl monomers onthe surface of the particles can be confirmed by, for example, a nuclearmagnetic resonance (NMR) measurement.

<Modified Natural Rubber Latex>

The modified natural rubber latex according to the present invention isproduced by dispersing the above modified natural rubber particleaccording to the present invention in an aqueous medium. By thefollowing production method, the modified natural rubber latex accordingto the present invention can be easily obtained.

<Production Method of Modified Natural Rubber Particles>

(1) The first production, method according to die present inventionincludes a step for forming an inclusion complex, a step for graftingand a step for deprotecting. Hereinafter, each step will be described inorder.

(a) Inclusion Complex Formation Step

This step is a step for forming an inclusion complex of themultifunctional vinyl monomers having two or more carbon-carbon doublebonds with a guest-protecting agent to protect at least one of thedouble bonds of the multifunctional vinyl monomers.

The multifunctional vinyl monomers are as described above. Therefore,here the description thereof is omitted. In this production method also,it is desired with respect to the multifunctional vinyl monomers toadopt the above-described advantageous modes. In addition, in thisproduction method, the reactivity of the carbon-carbon double bonds inthe multifunctional vinyl monomers used is not considered. In otherwords, all of reactivities of the carbon-carbon double bonds may be thesame or a part of the reactivities may be different.

As the guest-protecting agent, for example cyclodextrin is suitable.Cyclodextrin includes α-cyclodextrin in which 6 glucopyranose units arebonded in a cyclic form, β-cyclodextrin in which 7 glucopyranose unitsare bonded in a cyclic form, and γ-cyclodextrin in which 8 glucopyranoseunits are bonded in a cyclic form. Among them, β-cyclodextrin isinexpensive and therefore preferred. In the center of cyclodextrin,there is a hole. It is hydrophobic in the hole and the carbon-carbondouble bond of the multifunctional vinyl monomers is included in thehole.

FIG. 1 shows a conceptual scheme of an inclusion complex formation withβ-cyclodextrin in the case where NDMA is used as the multifunctionalvinyl monomers. When FIGS. 1 to 3 are summarized, it becomes aconceptual scheme of a series of steps in this production method. Asshown in FIG. 1, NDMA is reacted with β-cyclodextrin. Then, thecarbon-carbon double bond at one side of the NDMA is included in thehole of β-cyclodextrin to form an inclusion complex.

The formation of an inclusion complex may be effected by droppingmultifunctional vinyl monomers into an aqueous solution of aguest-protecting agent at a predetermined temperature. At this time, theadditive amount of the guest-protecting agent relative to themultifunctional vinyl monomers depends on the number of thecarbon-carbon double bonds to be protected. However, for example, theadditive amount may be guest-protecting agent; multifunctional vinylmonomers=around 1 to 2:1 in the molar ratio. After the completion, ofthe reaction, the inclusion complex may be filtered off, cleaned withdistilled water, and dried.

(b) Grafting Step

This step is a step for graft-copolymerizing the above-formed inclusioncomplex onto the natural rubber particles or the deproteinized naturalrubber particles.

The natural rubber particles or the deproteinized natural rubberparticles are as described above. Therefore, here the descriptionthereof is omitted. In this step, a polymerization initiator is added tothe latex of natural rubber or deproteinized natural rubber prepared toa predetermined concentration and then, to the resultant mixture, theformed inclusion complex is added to be graft-copolymerized.

Examples of the polymerization initiator include peroxides such aspotassium peroxide (KPS), ammonium persulfate (APS), benzoyl peroxide(BPO), hydrogen peroxide, cumene hydroperoxide, tert-butyl hydroperoxide(TBHPO), di-tert-butyl peroxide and 2,2-azobisisobutyronitrile (AIBN).From the viewpoint of lowering the polymerization temperature, aredox-type polymerization initiator may be used. Examples of a reducingagent used in combination with the above peroxides for the redox typepolymerization initiator include tetraethylenepentamine (TEPA),mercaptens, sodium hydrogen sulfite, a reductive metal ion and ascorbicacid. Examples of the suitable combination as the redox-typepolymerization initiator include TBHPO and TEPA, hydrogen peroxide andFe²⁺ salts, and KPS and sodium hydrogen sulfite. The additive amount ofthe polymerization initiator may be 0.01 to 0.3 mol relative to 1 kg ofthe amount of the rubber content (dry rubber weight, the same asfollows) in a latex of natural rubber etc.

In the latex of natural rubber etc., an emulsifying agent may also beadded beforehand. As the emulsifying agent, any of known various anionicsurfactants, nonionic surfactants and cationic surfactants can be used.Specifically, it is desired to control the surfactant in a range of pH 6to 13. Examples of the anionic surfactant include carboxylic acid-based,sulfonic acid-based, and sulfate ester-based surfactants. Examples ofthe nonionic surfactant include polyoxyalkylene ether-based andpolyalcohol aliphatic acid ester-based surfactants. Examples of thecationic surfactant include alkylamine salt-type and imidazolimumsalt-type surfactants. For example, an anionic surfactant such as sodiumdodecylsulfate is preferred.

The additive amount of the inclusion complex is desirably 0.01 to 3 molrelative to 1 kg of the amount of the rubber content in the latex of anatural rubber etc. When the additive amount of the inclusion complex(that is, multifunctional vinyl monomers) is more than 3 mol, aconcretion may be generated during the formation. On the contrary, whenthe additive amount is less than 0.01 mol, the grafted amount of themultifunctional vinyl monomers becomes small and desired effects ofmodification cannot be obtained.

FIG. 2 shows a conceptual scheme of the graft-copolymerization reactionof the deproteinized natural rubber particles with the inclusion complexformed in the above step (refer to FIG. 1). An initiator is added to adeproteinized natural rubber latex and then, an inclusion complex isadded thereto. Then, as shown in FIG. 2, utilizing non-protectedcarbon-carbon double bond, the inclusion complex is graft-copolymerizedonto the surface of the deproteinized natural rubber particles.

The polymerization reaction may be effected at 30 to 90° C. for 0.5 to12 hours. The obtained graft-copolymer is dried and then subjected tothe next deprotection step.

(c) Deprotection Step

This step is a step for removing the guest-protecting agent from theobtained graft-copolymer. When cyclodextrin is used as aguest-protecting agent, for example, distilled water is added to thegraft-copolymer, and the resultant mixture is stirred at a temperatureof 60° C. or more, so that cyclodextrin is dissolved and removed. Afterthe guest-protecting agent is removed, the product is dried to obtainthe modified natural rubber particle having the carbon-carbon doublebonds assigned to the multifunctional vinyl monomers graft-copolymerizedonto the surface thereof.

FIG. 3 shows a conceptual scheme of the present step. As shown in FIG.3, by dissolving and removing β-cyclodextrin from the graft-copolymerobtained in the above step, the modified natural rubber particle havingthe carbon-carbon double bonds derived from NDMA on the surface thereofcan be obtained.

(2) The second production method of the present invention is a methodfor graft-copolymerizing multifunctional vinyl monomers having two ormore carbon-carbon double hoods, at least one of which is distinguishedin reactivity from the other double bonds, onto natural rubber particlesor deproteinized natural rubber particles.

The natural rubber particles or the deproteinized natural rubberparticles are as described above. Therefore, the description thereof ishere omitted. In addition, in the present production method, themultifunctional vinyl monomers used are limited. In other words, itbecomes a condition that in the above-described multifunctional vinylmonomers, among a plurality of carbon-carbon double bonds, at least onedouble bond is distinguished in reactivity from the other double bonds.Examples of the multifunctional vinyl monomers capable of being usedinclude GAM and allyloxypolyethylene glycol-polypropylene glycolmono(meth)acrylate.

A double bond having a low reactivity tends to remain without beingreacted during the graft-copolymerization. Therefore, by the presentproduction method, the modified natural rubber particle having thecarbon-carbon double bonds assigned to the multifunctional vinylmonomers graft-copolymerized onto the surface thereof can be obtainedwithout protecting the double bonds. Accordingly, except thatmultifunctional vinyl monomers are used as they are instead of aninclusion complex, the graft-copolymerization may be performed accordingto the grafting step of the first production method according to thepresent invention.

Examples

Each of 4 types of multifunctional vinyl monomers was copolymerized ontothe deproteinized natural rubber particles to produce the modifiednatural rubber particles. The description thereof will be made in order.

<Production of Deproteinized Natural Rubber Latex>

As a natural rubber latex, a high ammonia latex (manufactured by GoldenHope Plantations Berhad (Malaysia); rubber content concentration: 60.2%by weight; ammonia concentration; 0.7% by weight) was used. First, thehigh ammonia latex was diluted to 30% by weight of rubber contentconcentration. Next, to the diluted latex, 1.0 part by weight of sodiumdodecyl sulfate (SDS: anionic surfactant) relative to 100 parts byweight of rubber content of the diluted latex was added to stabilize thelatex. Next, 0.1 parts by weight of urea relative to 100 parts by weightof rubber content of the latex was added to the latex and the resultantmixture was subjected to a protein decomposition treatment by stirringthe resultant mixture at room temperature for one hour. Subsequently,the latex with which protein decomposition treatment had been completedwas subjected to a centrifugation treatment at 10,000 rpm for 30minutes. A cream content of the thus separated upper layer was dispersedin water or an SDS aqueous solution to a predetermined rubber contentconcentration to obtain a deproteinized natural rubber latex.

<Grafting by NDMA>

Using NDMA (refer to the structural formula (2)) as the multifunctionalvinyl monomer, the modified natural rubber particles were produced bythe first production method according to the present invention. As theguest-protecting agent, β-cyclodextrin was used.

(1) Formation of Inclusion Complex

First, a β-cyclodextrin aqueous solution containing 0.11% by weight ofβ-cyclodextrin was charged into a separable flask and the inside of theflask was subjected to nitrogen-purge in a water bath for one hour toremove dissolved oxygen in the β-cyclodextrin aqueous solution. Next,while stirring the β-cyclodextrin aqueous solution at a constanttemperature of 90° C., NDMA was dropped into the aqueous solution andthe reaction was effected for 5 hours. The molar ratio of β-cyclodextrinand NDMA was set to 1:1. After the completion of the reaction, aninclusion complex was filtered off by a suction filtration. Theinclusion complex was cleaned with distilled water and vacuum-dried at50° C. for one week.

(2) Graft-Copolymerization

A deproteinized natural rubber latex prepared to a predetermined rubbercontent concentration was charged into a stainless steel-made containerand the inside of the container was subjected to nitrogen-purge for onehour to remove dissolved oxygen in the latex. While stirring the latexat a predetermined temperature, a polymerization initiator was droppedinto the latex and the resultant mixture was stirred for 5 minutes,followed by further adding the inclusion complex in a powder form to themixture. The additive amount of the inclusion complex was set to 0.1 molrelative to 1 kg of the rubber content. The reaction was effected for apredetermined time and the latex was transferred to a petri dish,followed by drying the latex at 50° C. for one week to obtain agraft-copolymer. In Table 1, the reaction conditions are summarized. InTable 1, the content rate of NDMA was obtained by measuring thegraft-copolymer after the deprotection by a Fourier transform infraredspectrophotometer (FT-IR). For the measurement FT/IR-410 (trade name;manufactured by JASCO Corporation) was used.

TABLE 1 Rubber Nitrogen content SDS Initiator Stiring Reaction ReactionNDMA introduction concentration concentration Polymerizationconcentration rate temperature time content method [%] [%] initiator*¹[mol/kg] [rpm] [° C.] [h] rate [%] Example 1 blowing 20 0.1 KPS 0.1 20060 5 1.20 Example 2 blowing 30 0 KPS 0.033 300 60 3 1.21 Example 3blowing 30 0.1 TBHPO 0.01 100 30 5 1.64 Example 4 blowing 30 1 BPO 0.1200 90 1 2.13 Example 5 blowing 30 0.1 TBHPO 0.1 200 40 1 1.74 *¹KPS:potassium persulfate, BPO: benzoyl peroxide, TBHPO: tert-butylhydroperoxide

(3) Deprotection (Removal of Guest-Protecting Agent)

The obtained graft-copolymer (solid) was cut into a square having a sizeof 20 mm×20 mm and the square graft-copolymer was put into a samplebottle. Distilled water was added into the sample bottle and the contentof the bottle was continuously stirred for 5 hours in a water bath at 70to 80° C. Distilled water was exchanged twice in the course of stirring.Subsequently, the graft-copolymer was air-dried and further vacuum-driedat 80° C. for 24 hours to obtain the graft-copolymer from which theguest-protecting agent was removed. Hereinafter, the producedgraft-copolymers will be referred to as copolymers of Examples 1 to 5corresponding to the numbers of Examples in Table 1.

(4) ¹H-NMR Measurement

The copolymers of Examples 1 to 5 were measured by a proton nuclearmagnetic resonance (¹H-NMR) measurement to confirm whether thecarbon-carbon double bond was present or not on the surface of themodified natural rubber particles of the copolymers. The measurement wasperformed using a nuclear magnetic resonance (NMR) apparatus (tradename: AL-400; manufactured by JEOL Ltd.). FIG. 4 shows, as one example,a ¹H-NMR spectrum for the copolymer of Example 2. In FIG. 4, (D) on thelast line is a spectrum for the copolymer of Example 2. As reference,(A) shows a spectrum for a deproteinized natural rubber; (B) shows aspectrum for NDMA; and (C) shows a spectrum for a graft-copolymerproduced by graft-copolymerizing NDMA without using a guest-protectingagent.

When comparing (C) and (D) in FIG. 4, in the spectrum (D), signalsappear at 5.50 ppm and 6.07 ppm. These signals are, as is apparent fromthe spectrum (B), signals ascribed of the carbon-carbon double bondsderived from NDMA. The similar signals do not appear in the spectrum(C). Thus, it has been confirmed that on the surface of the copolymer ofExample 2, the carbon-carbon double bonds derived from NDMA werepresent. Here, though it is omitted, to refer to the ¹H-NMR spectrum,also in the copolymers of Examples 1 and 3 to 5, it was confirmed in asimilar manner that the carbon-carbon double bonds derived from NDMAwere present. Thus, by protecting the carbon-carbon double bond at oneside of the NDMA, the graft-copolymerization can be performed whileretaining the carbon-carbon double bonds.

(5) Evaluation of Tensile Property and Viscoelasticity

With respect to the copolymers of Examples 1 to 5, the tensile strength(TS) and the elongation at break (E_(b)) were measured. Thesemeasurements were performed according to the Japanese IndustrialStandard (JIS) K 6251 (2004). Here, test pieces in a dumbbell shapethird form were used. In addition, the copolymers of Examples 1 to 5were molded into predetermined forms and were photocrosslinked byirradiating ultraviolet (UV) rays thereto for 2 minutes. Also, withrespect to each sample after the crosslinking, the tensile strength andthe elongation at break were measured in a similar manner. Themeasurement result is shown in Table 2. In Table 2, the rate of changeis a value calculated according to tire following equation: (Rate ofchange (%)=(value after crosslinking−value before crosslinking)/valuebefore crosslinking). Here, in Table 2, for comparison, the values ofthe tensile strength and the elongation at break of the deproteinizednatural rubber as a raw material are shown together.

TABLE 2 Tensile strength Elongation at break before after before afterNDMA photo- photo- rate of photo- photo- rate of content crosslinkingcrosslinking change crosslinking crosslinking change rate [%] [MPa][MPa] [%] [%] [%] [%] Example 1 1.20 0.54 0.78 45 477 287 −40 Example 21.21 0.80 0.89 11 308 108 −65 Example 3 1.64 0.71 1.66 135 197 202 3Example 4 2.13 0.41 1.23 204 526 260 −51 Example 5 2.63 0.82 1.68 105250 180 −28 Deproteinized 0.26 — — 525 — — natural rubber

As is apparent from Table 2, the tensile strengths of copolymers ofExamples 1 to 5 were increased by the crosslinking. On the other hand,since the carbon-carbon double bonds in the graft-copolymerized NDMAwere reacted by the crosslinking, die elongations at break were reduced.Evaluating the measurement results of the tensile strength and theelongation at break together, it is considered that the crosslinking wasprogressed.

Next, with respect to the copolymers of Examples 1 to 5 (including thecopolymers after the photocrosslinking), the relationship between thetemperature and the storage normal modulus was measured. First, fromeach copolymer, a test piece in a strip form having a width of 5 mm anda thickness of 1 mm was produced. Next, the produced test piece was setin a dynamic viscoelasticity measuring apparatus (trade name: Rheogel-E4000; manufactured by UBM Co., Ltd.) to measure the relationship betweenthe temperature and the storage normal modulus. The measurement wasperformed in a tensile mode under the condition in which a frequency was1 Hz and a displacement was 10 μm. FIGS. 5 to 7 show the relationshipsbetween the temperature and the storage normal modulus (E′) forcopolymers of Examples 1 to 3. In FIGS. 5 to 7, “E+05” in the ordinatemeans “×10⁵”. Therefore, for example, “1.00×10⁵” is expressed as“1.00E+05”.

As shown, in FIGS. 5 to 7, the storage normal modulus of any copolymerexhibited a similar tendency as that of the deproteinized natural rubberas a raw material. In addition, the storage normal modulus of copolymersof Examples 1 to 3 were slightly higher than that of the deproteinizednatural rubber. This indicates that the structures of the copolymers ofExamples 1 to 3 became specific structures by the grafting. In addition,from the fact that by a UV ray irradiating treatment, the graph isshifted to the side of higher temperatures and the storage normalmodulus is increased, it can be determined that the photocrosslinkingreaction of the carbon-carbon double bonds in the graft-copolymerizedNDMA was progressed.

<Grafting by GAM>

Using GAM (refer to the structural formula (3)) as the multifunctionalvinyl monomer, one of the modified natural rubber particles wereproduced by the first production method according to the presentinvention (Example 8), and other two of the modified natural rubberparticles were produced by the second production method according to thepresent invention (Examples 6 and 7). In the first production method,β-cyclodextrin was used as the guest-protecting agent.

(1) Formation of Inclusion Complex

First, a β-cyclodextrin aqueous solution containing 0.11% by weight ofβ-cyclodextrin was charged into a separable flask and the inside of theflask was subjected to nitrogen-purge in a water bath for one hour toremove dissolved oxygen in the β-cyclodextrin aqueous solution. Next,while stirring the β-cyclodextrin aqueous solution at a constanttemperature of 90° C., GAM was dropped into the aqueous solution and thereaction was effected for 5 hours. The molar ratio of β-cyclodextrin andGAM was set to 1:1. After the completion of the reaction, an inclusioncomplex was filtered off by a suction filtration. The inclusion complexwas cleaned with distilled water and vacuum-dried at 50° C. for oneweek.

(2) Graft-Copolymerization

A deproteinized natural rubber latex prepared to a rubber contentconcentration of 10% by weight was charged into a stainless steel-madecontainer and the inside of the container was subjected tonitrogen-purge for one hour to remove dissolved oxygen in the latex.While stirring the latex at a predetermined temperature, apolymerization initiator was dropped into the latex and the resultantmixture was stirred for 5 minutes, followed by further adding theinclusion complex in a powder form, or GAM as it is, to the mixture. Theadditive amounts of both the inclusion complex and GAM were set to 0.15mol relative to 1 kg of the rubber content. The reaction was effectedfor about one hour and then the latex was transferred to a petri dish,followed by drying the latex at 80° C. for 17 hours to obtain agraft-copolymer. In Table 3, the reaction conditions and the like aresummarized.

TABLE 3 Rubber Nitrogen content SDS Initiator Stirring Reaction Reactionintroduction concentration concentration Polymerization concentrationrate temperature time method [%] [%] initiator*¹ [mol/kg] [rpm] [° C.][h] Example 6 bubbling 10 0.1 TBHPO/TEPA 0.033 200 30 1 Example 7bubbling 10 0.1 KPS 0.033 200 75 1 Example 8*² bubbling 10 0.1TBHPO/TEPA 0.033 200 30 1 *¹KPS: potassium persulfate, TBHPO: tert-butylhydroperoxide, TEPA: tetraethylene pentamine *²Guest-protecting agent isused.

(3) Deprotection (Removal of Guest-Protecting Agent)

The graft-copolymer (solid) synthesized using a guest-protecting agentwas cot into a square having a size of 20 mm×20 mm and the squaregraft-copolymer was put into a sample bottle. Distilled water was addedinto the sample bottle and the content of the bottle was continuouslystirred for 5 hours in a water bath of 70 to 80° C. Distilled water wasexchanged twice in the course of stirring. Subsequently, thegraft-copolymer was air-dried and further vacuum-dried at 80° C. for 24hours to obtain the graft-copolymer from which the guest-protectingagent was removed. Hereinafter, the produced graft-copolymers will bereferred to as copolymers of Examples 6 to 8 corresponding to thenumbers of Examples in Table 3.

(4) ¹H-NMR Measurement

The copolymers of Examples 6 to 8 were measured by a ¹H-NMR measurementto confirm whether the carbon-carbon double bond was present or not onthe surface of the modified natural rubber particles of the copolymers.The measurement was performed using an NMR apparatus (trade name:AL-400; manufactured by JEOL Ltd.). FIG. 8 shows, as one example, a¹H-NMR spectrum for the copolymer of Example 7. In FIG. 8, (C) on thelast line is a spectrum for the copolymer of Example 7. As reference,(A) shows a spectrum for a deproteinized natural rubber and (B) shows aspectrum for GAM.

As shown in FIG. 8, in the spectrum (C), signals appear at 5.58 ppm and6.15 ppm. These signals are ascribed of the carbon-carbon double bondsin the acrylate in GAM. On the other hand, signals appear in thespectrum (B) at 5.80 ppm and 6.43 ppm. These signals are ascribed of thecarbon-carbon double bonds in the methacrylate. However, these signalsdo not appear in the spectrum (C). As is apparent from this result, thecarbon-carbon double bond in the methacrylate was preferentially reactedand the carbon-carbon double bond in the acrylate remained.

Thus, it has been confirmed that on the surface of the copolymer ofExample 7, the carbon-carbon double bonds derived from GAM were present.Here, though it is omitted to refer to the ¹H-NMR spectrum, also in thecopolymers of Examples 6 and 8, it was confirmed in a similar mannerthat the carbon-carbon double bonds derived from GAM were present. Thus,when GAM having carbon-carbon double bonds having different reactivitiesis used, the graft-copolymerization can be performed while retaining thecarbon-carbon double bond even without using a guest-protecting agent.

(5) Evaluation of Tensile Property

With respect to the copolymers of Examples 6 to 8, the tensile strengthand the elongation at break were measured. These measurements wereperformed according to JIS K 6251 (2004) as in Examples 1 to 5. Testpieces in a dumbbell shape third form were used. The measurement resultis shown in Table 4. In Table 4, for comparison, the values of thetensile strength and the elongation at break of the deproteinizednatural rubber as a raw material are shown together.

TABLE 4 Tensile Elongation strength at break [Mpa] [%] Example 6 1.04275 Example 7 0.80 200 Example 8 0.89 210 Deproteinized 0.26 525 naturalrubber

As shown in Table 4, the tensile strengths of the copolymers of Examples6 to 8 were increased than that of the deproteinized natural rubberbefore the modification. In addition, when touching each copolymer withfingers, tack property particular to the natural rubber was reduced andthe copolymer became hard. From this result, it can be determined that,as in the case of NDMA, the grafting was progressed.

<Grafting by Glycidyl Undecylate Methacrylic Acid-Adduct>

Using a glycidyl undecylate methacrylic acid-adduct (refer to thestructural formula (4)) as the multifunctional vinyl monomer, themodified natural rubber particles were produced by the second productionmethod according to the present invention.

First, a deproteinized natural rubber latex prepared to a rubber contentconcentration of 20% by weight was charged into a separable flask andthe inside of the flask was subjected to nitrogen-purge for one hour toremove dissolved oxygen in the latex. Next, while stirring the latex ata temperature of 60° C., KPS as a polymerization initiator was droppedinto the latex and the resultant mixture was stirred for 5 minutes,followed, by further adding glycidyl undecylate methacrylic acid-adductto the mixture. The additive amount of glycidyl undecylate methacrylicacid-adduct was set to 0.1 mol relative to 100 parts by weight of therubber content. The reaction was effected for about 3 hours and then therubber content was recovered by a centrifugation, followed byvacuum-drying the rubber content at 50° C. for one week to obtain agraft-copolymer. Hereinafter, the produced graft-copolymer will bereferred to as the copolymer of Example 9.

The copolymer of Example 9 was measured by a ¹H-NMR measurement toconfirm whether the carbon-carbon double bond was present or not on thesurface of the modified natural rubber particles of the copolymer. Themeasurement was performed using an NMR apparatus (trade name: AL-400;manufactured by JEOL Ltd.). As a result, in the spectrum for thecopolymer of Example 9, both the signals (at 5.6 ppm and 6.1 ppm)ascribed of the carbon-carbon, double bonds in the methacrylate and thesignal (at 4.9 ppm) ascribed of the carbon-carbon double bonds in theallyl group in the glycidyl undecylate methacrylic acid-adduct appeared.Further, by comparing the integrated intensities of both signals witheach other, the ratio between remaining amounts of methacrylate andallyl group was calculated. Then, it became apparent that the allylgroup remained in an amount more than two times the amount of remainingmethacrylate. As is apparent from this result, the carbon-carbon doublebond in the methacrylate was preferentially reacted and thecarbon-carbon double bond in the allyl group remained. Thus, it has beenconfirmed that on the surface of the copolymer of Example 9, thecarbon-carbon double bonds derived from the glycidyl undecylatemethacrylic acid-adduct were present. Thus, when glycidyl undecylatemethacrylic acid-adduct having carbon-carbon double bonds with differentreactivities is used, the graft-copolymerization can be performed whileretaining the carbon-carbon double bond even without using aguest-protecting agent.

<Grafting by Divinylbenzene>

Using divinylbenzene as the multifunctional vinyl monomer, the modifiednatural rubber particles were produced by the first production methodaccording to the present invention, β-cyclodextrin was used as theguest-protecting agent.

(1) Formation of Inclusion Complex

First, a β-cyclodextrin aqueous solution containing 0.11% by weight ofβ-cyclodextrin was charged into a separable flask and the inside of theflask was subjected to nitrogen-purge in a water bath by a blowingmethod for one hour to remove dissolved oxygen in the β-cyclodextrinaqueous solution. Next while stirring the β-cyclodextrin aqueoussolution at a constant temperature of 90° C., divinylbenzene was droppedinto the aqueous solution and the reaction was effected for 5 hours. Themolar ratio of β-cyclodextrin and divinylbenzene was set to 1:1. Afterthe completion of the reaction, an inclusion complex was filtered off bya suction filtration. The inclusion complex was cleaned with distilledwater and vacuum-dried at 50° C. for one week.

(2) Graft-Copolymerization

A deproteinized natural rubber latex prepared to a rubber contentconcentration of 30% by weight by dispersing a deproteinized naturalrubber in a SDS aqueous solution containing 0.1% by weight of SDS, wasused. First, the deproteinized natural rubber latex was charged into astainless steel-made container and the inside of the container wassubjected to nitrogen-purge by a blowing method for one hour to removedissolved oxygen in the latex. Next, while stirring the latex at 30° C.,tert-butyl hydroperoxide (TBHPO) and tetraethylene pentamine (TEPA) asthe polymerization initiators were dropped into the latex in order. Thetotal additive amount of the polymerization initiator was set to 0.07mol relative to 1 kg of the rubber content. Next, the inclusion complexin a powder form was added, thereto. The additive amount of theinclusion complex was set to 0.1 mol relative to 1 kg of the rubbercontent. The reaction was effected for about 2 hours and then the latexwas transferred to a petri dish, followed by drying the latex at 50° C.for one week to obtain a graft-copolymer.

(3) Deprotection (Removal of Guest-Protecting Agent)

The obtained graft-copolymer (solid) was cut into a square having a sizeof 20 mm×20 mm and the square graft-copolymer was put into a samplebottle. Distilled water was added into the sample bottle and the contentof the bottle was continuously stirred for 5 hours in a water bath of 70to 80° C. Distilled water was exchanged twice in the course of stirring.Subsequently, the graft-copolymer was air-dried and further vacuum-driedat 80° C. for 24 hours to obtain the graft-copolymer from which theguest-protecting agent was removed. Hereinafter, the producedgraft-copolymer will be referred, to as the copolymer of Example 10.

(4) Evaluation of Tensile Property

With respect to the copolymer of Example 10, the tensile strength andthe elongation at break were measured. These measurements were performedaccording to JIS K 6251 (2004) as in Examples 1 to 8. Test pieces in adumbbell shape third form were used. The measurement result is shown inTable 5. In Table 5, for comparison, the values of the tensile strengthand the elongation at break of the deproteinized natural rubber as a rawmaterial are shown together.

TABLE 5 Tensile Elongation strength at break [Mpa] [%] Example 10 0.48483 Deproteinized 0.26 525 natural rubber

As shown in Table 5, the tensile strength of the copolymer of Examples10 was increased than that of the deproteinized natural rubber beforethe modification. In addition, when touching the copolymer of Example 10with fingers, tack property particular to the natural rubber was reducedand the copolymer became hard. From this result, it can be determinedthat, as in the case of NDMA and GAM, the grafting was progressed.

(5) Observation by Scanning Probe Microscope (SPM)

The copolymer of Example 10 and the deproteinized natural rubber as araw material were observed by an SPM (observation range: 10 μm square).FIG. 9 shows an SPM image of the deproteinized natural rubber. FIG. 10shows an SPM image of the copolymer of Example 10.

When FIG. 9 and FIG. 10 are compared, in the copolymer of Example 10(FIG. 10), the surrounding of the rubber particles is seen dark. Thatis, in the copolymer of Example 10, it is apparent that divinylbenzene(multifunctional vinyl monomer) is bonded to the surrounding of thedeproteinized natural rubber particles.

INDUSTRIAL APPLICABILITY

In the surface of the modified natural rubber particle according to thepresent invention, the carbon-carbon double bonds in thegraft-copolymerized multifunctional vinyl monomers remain. Thecarbon-carbon double bonds become reactive sites for crosslinking etc.Accordingly, for example, by photocrosslinking etc., a three-dimensionalnetwork structure can be formed in a grafted-polymer layer in a surfaceof the modified natural rubber particle. Thus, the tensile property, thedimensional stability etc. of rubber materials after the crosslinkingare Improved. Such modified natural rubber particle according to thepresent invention is useful in applications, such as a rubber vibrationinsulator, as an alternative material of a synthetic rubber.

What is claimed is:
 1. A method for producing modified natural rubberparticle having carbon-carbon double bonds of vinyl structures assignedto multifunctional vinyl monomers as a source for graft-copolymerizationon a surface of the modified natural rubber particle, comprising:graft-copolymerizing at least one multifunctional vinyl monomer havingtwo or more carbon-carbon double bonds of vinyl structures, none of thecarbon-carbon double bonds being protected by cyclodextrins, at leastone of the carbon-carbon double bonds being distinguished in reactivityfrom the other carbon-carbon double bonds, and selected from the groupconsisting of 3-acryloyloxy-2-hydroxypropyl methacrylate, glycidylundecylate methacrylic acid-adduct, allyloxy polyethyleneglycol-polypropylene glycol mono(meth)acrylates, allyloxy polyalkyleneglycol mono(meth)acrylates, allyloxy polyalkyl alcoholmono(meth)acrylates, (vinylphenyl)allylalkanes, and(vinylphenyl)allylalkylene glycols, onto natural rubber particles ordeproteinized natural rubber particles.